I don't understand (help please)

If So-hee got her wish by killing Jin-sung and dragging her back with her, why would she return at the end to keep on haunting what used to be Jin-sung's room? Even though it would be a stupid ending, it would have made more sense if Jin-sung came back with her since So-hee was so dependant on her presense.

When the frustrated girl wished to win the art competition, how is being killed by the supposedly possessed Hae-ju and made into a 'sculpture' helping her win? It would've been different if she wished that she was beautiful as her artwork of if she wished that she could make a prize-winning artpiece, but then Jae-ju wouldn't have to kill her to if that were the case... Whatever.
Was Hae-ju really possessed in the scene where Jin-sung says that if she were So-hee she would know that she didn't push her down the stairs?- I got super-confused on that.

If someone could clear this up for me I would really appreciate this.